1 Units & Measurements

Question 1

What is a unit?

Answer 1

• Measurement of any physical quantity involves comparison with a certain basic and internationally accepted reference standard called unit.

Question 2

What are fundamental units?

Answer 2

The units for the fundamental base quantities are called fundamental or base units

Question 3

What are derived units?

Answer 3

• The units of all other physical quantities can be expressed as combinations of base units. Such units are called derived units

Question 4

What are the three systems of units?

Answer 4

Three systems of units – the CGS, the FPS (British) and the MPS system

Question 5

Who developed the International System of Units?

Answer 5

• This system was developed by The International Bureau of Weights and Measures (BIPM) in 1971 which was recently revised by the GCWM in 2018

Question 6

What is the parallax formula?

Answer 6

θ = l/r

Question 7

What are the units for plane and solid angles?

Answer 7

The unit for plane angle is radian with the symbol ‘rad’ and the unit for the solid angle is steradian with the symbol ‘sr’. Both these quantities are dimensionless

Question 8

Which method is used to estimate the sizes of molecules?

Answer 8

Tunnelling Microscopy

Question 9

What is parsec?

Answer 9

• Parsec is the distance at which average radius of Earth’s orbit subtends an angle of 1 arc sec

1 parsec = 3.0 x 10^16 m

Question 10

What is 1 light year?

Answer 10

1 ly = 9.46 x 10^15 m

Distance that light travels with the velocity of 3 x 10^8 m/s in 1 year

Question 11

What is 1 AU?

Answer 11

1 AU = Average distance of the Sun from the Earth

1.4 x 10^11 m

Question 12

What is 1 fermi?

Answer 12

1 fermi = 10^-15 m

Question 13

What is 1 angstrom?

Answer 13

1 angstrom = 10^-10 m

Question 14

Which authority is responsible for maintenance of Indian Standard Time?

Answer 14

A caesium atomic clock is used at the National Physical Laboratory (NPL), Delhi to maintain the Indian Standard Time

Question 15

What is the difference between precision and accuracy?

Answer 15

• The accuracy of measurement is a measure of how close the measured value is to the true value of the quantity. It depends on several factors such as the limit or resolution of the measuring instrument.
• Precision tells us to what resolution or limit the quantity is measured.

Question 16

What are systematic errors?

Answer 16

• The systematic errors are those that tend to be in one direction, either positive or negative

Question 17

What are instrumental errors?

Answer 17

• Instrumental Errors that arise from the errors due to imperfect design or calibration of the measuring instrument, zero error, etc.

Question 18

What is least count?

Answer 18

• The smallest value that can be measured by the measuring instrument is called its least count.
• The least count error is the error associated with resolution of instrument. It belongs to the category of random errors within a limited size.
• Using precise instruments, improving experimental techniques, etc., least count error can be reduced. Repeating the observations and then taking average of all observations would be very close to the true value of the measured quantity

Question 19

What is absolute error?

Answer 19

• The magnitude of difference between the individual measurement and true value of the quantity is called the absolute error of the measurement.

The ‘a’ calculated may be positive or negative but we take only the positive value

Question 20

What is the formula of relative error?

Answer 20

Relative Error = dA(mean)/A(mean)

It is the ratio of mean absolute error to the mean value of observations.

Question 21

What are the rules for combination of errors in addition and subtraction?

Answer 21

When two quantities are added or subtracted, the absolute error in the final result is the sum of the absolute errors in the individual quantities

Question 22

What is the formula for relative error for multiplication/division/exponent?

Answer 22

dZ/Z = p dA/A + q dB/B + r dC/C

Question 23

What are the rules for addition/subtraction of significant figures?

Answer 23

• In addition or subtraction, the final result should retain as many decimal places as are there in the number with least decimal places

Question 24

What are the rules for multiplication/division of significant figures?

Answer 24

• In multiplication or division, the final result should retain as many significant figures as there are in the original number with the least significant figures

Question 25

What is the Principle of Homogeneity?

Answer 25

According to the Principle of Homogeneity, dimensions of each of the terms of a dimensional equation on both sides should be the same

Question 26

What is the formula of Least Count of Vernier Calliper?

Answer 26

LC = MSD (1 - m/n)

Question 27

What is the formula of total reading of Vernier Calliper?

Answer 27

Total Reading = MSR + (LC x Coinciding Division)

Question 28

What is the formula for Least Count of Screw gauge?

Answer 28

LC = Pitch/No. of Circular Divisions

Question 29

Light year is the unit of ____________.

Answer 29

Distance

Question 30

____________ is used to measure the mass of atoms and molecules.

Answer 30

Mass Spectrograph

Question 31

Checking the correctness of equations using the method of dimensions is based on _______________________________.

Answer 31

Principle of Homogeneity of Dimensions